Independent origins of frog glue and its role in predator escape

Skin-secreted adhesives, or glues, are highly effective defensive adaptations that have evolved repeatedly in a small number of amphibians. From an ecological perspective, this rapidly solidifying material (essentially a slime) encumbers the predator long enough for its potential prey to escape.

But what makes some skin secretions stickier than others, and why has this happened multiple times throughout amphibian evolutionary history?

Adhesives in Nature: Ancient Tools for Survival

There’s no denying that the materials that allow us to stick things to other things—namely, glue—are ubiquitous. You might find yourself reaching for a stack of post-its or a roll of duct tape, which always works. But what about glue in other, less-adept animals? What is it for, and how does it work?

Before we go any further, let’s clarify what exactly is meant by “animal glue.” In the context of glue-producing organisms, the materials I’m referring to are called “biological adhesives.” These are naturally secreted materials found in a wide range of species, many of which perform vital functions necessary for the survival of that particular organism.

The potential uses of these glues are as diverse as the animals that produce them, and include substrate attachment (typical of sessile marine organisms such as mussels, barnacles and tubeworms), locomotion (used by starfish to move along the ocean floor) and prey capture (most easily recognizable in the form of spider silk).

Indeed, the sheer utility and adaptive value of glue is reflected in its wide taxonomic distribution, spanning several divergent ancient lineages.

There’s one thing you may have noticed: all the animals I’ve described so far are invertebrates. What about species that look a little more like our animals, like a tetrapod?

If you ever find yourself in the company of herpetologists, simply mention the words “sticky” and “frog” and you’ll inevitably hear about that time a wild specimen oozed copious amounts of slime all over the offender’s hands.

This slimy substance quickly took on the properties of superglue: the hands stuck to each other, the frog stuck to the hands – in short, a very sticky situation. This may be mere anecdotal evidence, but if you look in the literature, you will find few mentions of this relatively obscure phenomenon. However, that does not make the stories any less true.

How Frogs Use Glue as a Defensive Strategy

Chemicals secreted by the skin are the most widespread antipredator adaptation in amphibians. In a small number of them, this mechanism occurs in the form of glue. When stressed, the amphibian releases a viscous liquid from its back that quickly solidifies into a sticky mass (i.e., glue).

This glue functions as an effective defensive weapon, incapacitating the attacking predator (often a snake) by clogging its mouth and making the act of swallowing impossible. The energetic cost of overcoming this viscosity eventually becomes too great, forcing the predator to give up and release the amphibian.

While toxic skin secretions (i.e., poisons) have long been the focus of most biochemical research, research on glue remains sparse and superficial. One possible reason for this disparity could be that glue is a rare trait in frogs, having emerged only sporadically in species that are evolutionarily distant from each other.

Although frog glue has been found all over the world, its absence in most species (and especially in their close relatives) is conspicuous. For example, a glue-producing frog in Madagascar might not share the island with glue-producing amphibians from other lineages.

In contrast, similar sticky secretions can be found in frogs whose distribution is limited to Australia or South America, for example.

This brings us to the heart of my study published in Nature Communications :How, exactly, did glue as a defense adaptation evolve in some frogs but not others?

And, by the way: does frog glue have anything in common with that duct tape you have at home?

The bonding process: from interdisciplinary to intermolecular

To answer these questions, I studied the glue produced by a species endemic to Madagascar: the tomato frog, Dyscophus guineti. In collaboration with several research institutions, I integrated functional, molecular, and evolutionary analyses to uncover what exactly makes frog glue sticky.

To this end, I have identified two proteins that demonstrably interact within the glue medium to maintain its adhesive and cohesive strength. One is a large glycoprotein (to which my thesis advisor has aptly given the acronym PRIT) whose role is presumed to be specific to glue and which contains duplicate copies of an evolutionarily conserved domain also present in many metazoan extracellular proteins.

The second, much smaller protein is a member of the ubiquitous glycan-binding protein family called galectin. These results are consistent with previous reports on the importance of glycoproteins and glycan-binding proteins in other animal glues, although their interactions and likely mechanism of action have not been resolved until recently.

Structural models predicted that while the conserved domains within PRIT are well defined, their intermediate regions are structurally heterogeneous. This contrasts with most (non-adhesive) proteins, which have rigid and defined structures, whereas the structural dynamism of PRIT makes it highly flexible.

In practical terms, this means that frog glue can adapt to any surface it comes into contact with, such as the oral epithelium of a snake. The transition from a viscous but fluid slime to a strong, fast-acting adhesive occurs once pressure is applied, such as the force exerted by a predator’s bite.

Back to our previous question: what do the humble duct tape and frog glue have in common? They are both pressure sensitive, meaning that a compressive force is required to fully “activate” their adhesive power.

A recipe for recurring evolution: reuse, recycle, re-evolve

With the identification of the glue proteins, I was finally able to begin to examine the genetic and structural changes that led to its evolution in distant lineages. As noted above, none of these proteins are inherently unique to D. guineti, or even to frogs in general. In fact, the protein domains found in frog glue are present in all animals, including humans.

The specific architecture of the PRIT gene, however, implies a deviation that evolved in an early amphibian ancestor. In other words, glue genes evolved before glue itself.

Interestingly, a second glue-producing species (the Mozambique rain frog, Breviceps mossambicus) also encodes a PRIT gene. Dyscophus and Breviceps diverged approximately 100 million years ago and belong to distinct frog radiations (Microhylidae and Afrobatrachia, respectively).

Other members of these lineages produce nonadhesive toxins that are known to have arisen early in frog evolution, leaving little doubt that: (1) Dyscophus and Breviceps both descended from a venomous ancestor; and (2) their skin secretions evolved independently into glues.

Along with structural changes, gene expression change was identified as a decisive factor in the recurrent evolution of glue: PRITs and galectins show the same high expression pattern in both glue-producing species, allowing us to speculate that regulatory changes also contributed to the parallel evolution of frog glue.

Unlike other glue-producing animals, each of which has evolved a unique method of adhesion, highly divergent frog lineages have repeatedly recruited the same pre-existing genes, simply by increasing their expression.

Frog glue is therefore the culmination of evolutionary processes that preceded it, with recurring structural and regulatory changes acting on an ancient and almost universal model.

From forest floors to operating tables: the future of biomimicry

My recent work represents the first detailed analysis of a vertebrate defense glue, advancing our understanding of these unusual adaptations while simultaneously opening the door to the development of new, fast-acting adhesive technologies.

The effectiveness of animal slime as surgical sealants has already been demonstrated using slug-defensive glue; for example, we now know how it works in a vertebrate, using a model with the potential to encompass biological glues from phylogenetically diverse sources.

Stay tuned: Soon, frog glue derivatives could become as crucial and common in surgical practices as duct tape is in our homes today.

This article is part of Science X Dialog, where researchers can present the results of their published research papers. Visit this page for information about Science X Dialog and how to participate.

From molecular wonders to the most majestic of them all, Shabnam Zaman has always sought to understand the “why” behind the whimsical. That’s how she found herself as a PhD researcher at the Amphibian Evolution Lab (Vrije Universiteit Brussel, Belgium) with a mission to investigate a strange but little-known phenomenon: frog glue. Together with her trusty companion, Bob the Tomato Frog, they embark on a quest to unravel the enduring mysteries of what makes these creatures so incredibly sticky.